I. Field of the Invention
This invention relates to exponential current generators and methods, and more particularly to generators used in variable gain amplifiers (VGA).
II. Description of the Related Art
As noted in the co-pending and related application cited above, variable gain amplifiers (VGA) may be employed to amplify received signals and signals to be transmitted. The gain of the VGA is determined by a control signal where the gain of the VGA in some applications is ideally exponentially (or, “linear in dB”) related to the control signal. The VGA may have an input stage and one or more current amplifier stages coupled to the input stage. The input stage receives a voltage signal, converts the voltage signal to current signals, and amplifies the current signals. The current amplifier stage further amplifies the current signals generated by the input stage.
Ideally, the gain of each current amplifier is exponentially related to the control signal. The control signal may be any type of control signal including a control current, digital control, or voltage signal, for example. In the co-pending and related application a voltage control signal, Vcontrol is employed to determine or control the gain or each current amplifier. As also described in the co-pending application, each current amplifier stage employs an exponential current generator that receives Vcontrol and generates a pair of current signals, Im and Ip, where the ratio of Ip/Im is exponentially related to the control signal Vcontrol. The pair Im, Ip is used to control the gain of the current amplifier as described in the co-pending application.
As noted above, a current amplifier stage is coupled to the input stage of the VGA. Due to certain circuit dynamics associated with coupling the input stage to a current amplifier stage, it is desirable that Im not fall below some minimum current value (“Ibleed”). As described in the related and incorporated co-pending application, in order to prevent Im from falling below a certain minimum current value, a current source is included to produce a fixed current Ibleed in parallel with Im. Adding Ibleed to Im ensures a minimum tail current in the input stage of the translinear loop of the current amplifier as described in the related application.
Disadvantageously, the addition of Ibleed to Im changes the ratio of the current pair produced by the exponential current generator to be proportional to Ip(Im+Ibleed). This ratio is disadvantageously not exponentially related to the control signal Vcontrol. This is particularly true as the ratio becomes large and Im becomes corresponding small, i.e., as Im approaches Ibleed. Disadvantageously, this distortion causes the gain of a current amplifier employing the generator to become more linearly related than exponentially related to Vcontrol. This distortion can create power control problems when the amplifier is used as part of a VGA that is used in a transmitter, in particular in a transmitter that generates a Code Division Multiple Access (CDMA) signal. Thus, a need exists for an improved exponential current generator that generates a current pair whose ratio is exponentially related to a control signal even when Im is less than Ibleed. The present invention provides such an improved exponential current generator.